In many applications involving the transmission of optical energy along an optical waveguide, such as an optical fiber, it is desirable to be able to project the best absolute transmission out of the optical waveguide without the need of actually measuring the transmission during operation. For example, the desired amount of energy to be coupled out of the optical probe tip for near-field scanning optical microscope (NSOM) machining is high enough to damage the substrate in the near-field region. Due to the high peak power level pulses used in this application, it is advantageous to use less than the full ablation power during the processes of optical alignment and optimization of the amount of light coupled into the NSOM probe. However, doing so reliably is not necessarily straightforward.
Conventionally, a fixed low power, substantially below the damage threshold of the optical waveguide, is used for alignment and the amount of light coupled into and transmitted through the optical waveguide is optimized at the fixed low power level. An optimized coupling and transmission efficiency may be obtained at this low power. It is assumed that this coupling and transmission efficiency is approximately the same for all power levels, until the fiber is damaged. Therefore, extrapolating the measured coupling and transmission efficiency to a high power input beam is assumed to yield high power output proportionally. However, experimental data, circles 400 and triangle 402 in FIG. 4, demonstrate the existence of a hard transmission saturation that is not revealed using a fixed low power optical beam and the constant-efficiency method described above. This data illustrates that the output power level may not track higher input power level when saturation occurs.
Another issue that may limit the amount of power that may be coupled into and transmitted through an optical waveguide is the potential for damage to the coupling surface of the optical waveguide. High power laser systems, both pulses and continuous wave (CW), may generate intensities high enough to machine, or otherwise damage, the surface of materials, even substantially transparent materials such as those used in optical waveguides. This potential for damaging the coupling surface is one reason that it may be desirable to optimize the optical coupling of optical waveguides at a lower power. Using lower power levels to optimize the optical coupling, however, may lead to alignment configurations in which the coupling surface of the optical waveguide is damaged when the optical beam is set to the desired input power level.
The present invention involves an improved method of optimizing the optical coupling of a high power optical beam into an optical waveguide. This improved method may increase the power level of the portion of the optical beam coupling into the optical waveguide as compared to other methods and may also reduce the potential for damaging the coupling surface of the optical waveguide.